1. Field of the Invention
The invention pertains to base band presence detection and range measurement systems and more particularly to means effectively narrowing the antenna patterns of such base band radio systems as employed in vehicular protective systems.
2. Description of the Prior Art
Systems employing sub-nanosecond base band pulses as object detectors generally exhibit beam widths too broad for good discrimination of angularly displaced targets. Good angular resolution is achieved in the more conventional pulse radar systems by the use of highly directive antennas. However, such antennas have relatively narrow pass bands. Antennas that are broad band enough to propagate sub-nanosecond pulses also have an inherently broad beam width. Where it is desired to detect targets within about two meters of the detector, the inherent broad beam widths produced by base band antennas are not a particular problem because the outer boundary of the range gate and the natural fall off of the antenna response severely restrict the area covered at the short range. For some distances beyond this range, signal processng techniques have been successfully employed to narrow the effective beam widths. For example, one such technique has been described in the U.S. Pat. No. 3,858,205 to G. F. Ross for a "Base-Band Precollision Sensor with Time Domain Gating for Preventing False Responses", issued Dec. 31, 1974 and assigned to Sperry Rand Corporation. However, such techniques become increasingly ineffective as the range to the target is greatly increased with respect to the separation of the transmitter and receiver antennas.
Generally speaking, it is difficult to obtain a system that exhibits both a narrow effective beam width and negligible signal dispersion (i.e., has a wide band width). In antenna array systems, for example, one can obtain a narrow beam by employing a multiplicity of elements and by making the aperture dimensions large compared to the center wave length. However, to obtain simultaneously a wide band width, it is necessary to employ real time delays in the element feed network rather than instantaneous phase shift behind each element to afford coherent addition at the appropriate sum port.
A further solution of the problem is set forth in the G. F. Ross U.S. Pat. No. 4,017,854, issued Apr. 12, 1977 for "Apparatus for Angular Measurement and Beam Forming With Baseband Radar Systems", assigned to Sperry Rand Corporation. In this latter system, there is described an electromagnetic energy pulse system for transmitting very short base band pulses and for the reception of such pulses by dual base band receivers, the receiving antennas being separated by a distance much less than the range to the target to be detected. The base band receivers amplify the received signals, the amplified signals being further processed by being fed to opposite ends of a tapped transmission line, whereupon the angle to the target may be determined by the tap at which the two pulses arrive in coincidence. The taps are spaced along the transmission line, whereupon the angle to the target may be determined by the tap at which the two pulses arrive in coincidence. The taps are spaced along the transmission line by distances somewhat greater than one-half the product of the signal velocity on the transmission line and the duration of the received and reconstituted pulses, thus providing a means by which the relative time delays between the two receiving antennas may be determined to within one half the pulse width and concomitantly establishing an effective radiant beam width for the system proportional to the ratio of the pulse duration to the distance separating the two receiver antennas. While representing a valuable improvement over the prior art, the system of U.S. Pat. No. 4,017,854 has certain difficulties that will be discussed in more detail hereinafter, especially when used for automatic braking in road vehicles. Such problems make the prior art devices generally unsuited for use, for example, for operating vehicular braking mechanisms with respect to roadway lane widths of about 2.0 meters at a 45 meter forward distance of the intruding object, since effective antenna pattern widths as narrow as 2.7 degrees are then needed. In such an example, symmetrically disposed receivers spaced by about 0.75 meters from the transmitter would be employed.